A mission critical push to talk (MCPTT) system can support a direct mode operation (DMO) scenario. For example, TS22.179 defines that MCPTT can support two DMO scenarios: one-to-one communication and private call. In an MCPTT system, a same user may not only be affiliated with multiple communications groups, but also use different MCPTT user equipments (UE) to acquire an MCPTT service. For example, in the MCPTT system shown in FIG. 1, a user 1 is affiliated with a communications group 1 and a communications group 2, and the user 1 may use MCPTT UE 1, MCPTT UE 2, and MCPTT UE 3 to obtain an MCPTT service.
In the MCPTT system, a user may use different MCPTT UEs to acquire an MCPTT service. Therefore, to ensure normal communication, two communication parties in a DMO scenario not only need to learn an application layer identifier, that is, a user identifier (user ID) of each other, but also need to learn a layer 2 identifier (a layer 2 ID), that is, a device identifier of MCPTT UE currently used by each other, so as to accurately identify each other, and then perform service communication in a proximity-based service (Proximity-based Services, Prose for short) manner.
However, in the prior art, when a source user needs to discover at least one target user, the source user generally uses the following discovery method to discover the target user: The source user sends a discovery message that includes a layer 2 identifier of MCPTT UE currently used by the user to the at least one target user, and after receiving the discovery message, the target user returns a response message that includes a layer 2 identifier of MCPTT UE currently used by the target user to the user. The source user cannot identify the target user according to the response message returned by the target user, and therefore the source user cannot perform service communication with the target user in the Prose manner.